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542458 • last Friday at 10:49 PM • 2 replies • view on HN

Okay, so I know back in the day you could choke scanning software (ie email attachment scanners) by throwing a zip bomb into them. I believe the software has gotten smarter these days so it won’t simply crash when that happens - but how is this done; How does one detect a zip bomb?

Replies

danudey • last Friday at 10:51 PM

I don't understand the code itself, but here's Debian's patch to detect overlapping zip bombs in `unzip`:

https://sources.debian.org/patches/unzip/6.0-29/23-cve-2019-...

    The detection maintains a list of covered spans of the zip files
    so far, where the central directory to the end of the file and any
    bytes preceding the first entry at zip file offset zero are
    considered covered initially. Then as each entry is decompressed
    or tested, it is considered covered. When a new entry is about to
    be processed, its initial offset is checked to see if it is
    contained by a covered span. If so, the zip file is rejected as
    invalid.
So effectively it seems as though it just keeps track of which parts of the zip file have already been 'used', and if a new entry in the zip file starts in a 'used' section then it fails.
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10000truths • last Friday at 11:38 PM

For any compression algorithm in general, you keep track of A = {uncompressed bytes processed} and B = {compressed bytes processed} while decompressing, and bail out when either of the following occur:

1. A exceeds some unreasonable threshold

2. A/B exceeds some unreasonable threshold

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